1. Field of the Invention
The invention relates to a process for making an oxazolidinone structures-containing prepolymeric epoxy resin mixture and to an apparatus for carrying out said process.
2. Description of Related Art
It is known from WO 90/15089 that epoxide-terminated polyoxazolidinones (in that document referred to simply as polyoxazolidones) can be prepared by reaction of a polyepoxide and a polyisocyanate at elevated temperature in the presence of a catalyst. To this end, from 5 to 30 wt % of the polyisocyanate is added within 30 to 90 min to a mixture of 70 to 95 wt % of the polyepoxide and 0.01 to 2 wt % of the catalyst, and the resulting reaction mixture is then heated at a temperature of 110.degree. to 200.degree. C. for a period of 5 to 180 minutes. By regulating various process parameters, the process is carried out so that in the resulting epoxy-terminated polyoxazolidinone, which is also referred to as isocyanate-modified epoxy resin, 50 to 100% of the original isocyanate groups are converted into oxazolidinone rings and 0 to 50% into isocyanurate rings.
In the known process, the polyepoxide is, in particular, bisphenol A or tetrabromobisphenol A, and the polyisocyanate is 4,4'-methylene-bis(phenyl isocyanate) (MDI) or an isomer thereof, polymeric MDI or toluylene diisocyanate. A suitable catalyst (for the reaction of the polyepoxide and the polyisocyanate) is, in particular, an imidazole or tetraphenylphosphonium bromide. The catalyst concentration is preferably from 0.02 to 1 wt %, particularly 0.02 to 0.1 wt %, based on the total weight of the polyepoxide and the polyisocyanate. To prepare the polyoxazolidinones, the catalyst, optionally dissolved in a suitable solvent, is added to the polyepoxide, in general at a temperature below the reaction temperature of 110.degree. to 200.degree. C. The temperature is then raised to the reaction temperature and kept at this level while adding the polyisocyanate under controlled conditions, namely dropwise.
A similar process, known from EP 0 296 450 A1, is used for making oxazolidinone groups--(in that document referred to simply as oxazolidone groups) containing oligomeric polyepoxides from bisepoxides and diisocyanates. By this process, either a bisepoxy ether with OH groups corresponding to a hydroxyl number of at least 2 is made to react with an aromatic diisocyanate containing two NCO groups of different reactivity in an amount of at least 1/4 of the weight of the diisocyanate, or a bisepoxy ester with OH groups corresponding to a hydroxyl number of at least 2 is made to react with an aromatic, aliphatic or cycloaliphatic diisocyanate in a weight ratio of NCO groups to epoxide groups of 1:1.4 to 1:2.5--both reactions being carried out in the presence of a phosphonium salt as catalyst at 140.degree. to 180.degree. C. The catalyst is used in an amount of 0.005 to 1.0 wt %, preferably 0.01 to 0.5 wt %, based on the bisepoxide.
In this process, it is essential that the oxazolidinone epoxy resins are obtained only when 0H groups-containing epoxy resins are made to react with diisocyanates containing NCO groups of different reactivity, in the presence of a phosphonium salt as catalyst at about 160.degree. C. To prepare the polyepoxide, the bisepoxy resin and the catalyst are heated to 160.degree. C. under nitrogen. The diisocyanate is then added dropwise to the melt at a rate such that a temperature of about 170.degree. C. is maintained. After all the diisocyanate has been added, the mixture is allowed to agitate at 160.degree. C. until the calculated epoxide content has been reached and reactive NCO can no longer be detected.
Both known processes have been described only for laboratory batch sizes. It is essential in this respect that the polyisocyanate be added dropwise to the catalyst-containing polyepoxide. Hence, it is hardly possible to carry out the described processes economically on an industrial scale. Moreover, by these processes only filler-free reaction resin mixtures can be used.